1. Field of the Invention
The present invention relates to a wavelength characteristic variable apparatus used to reduce the deviation of the signal strength of each wavelength caused in an optical fiber transmission line, an optical amplifier, etc. for use in a long distance wavelength-multiplexed transmission device.
2. Description of the Related Art
In a large capacity long distance transmission device, the number of optical amplification repeaters is increased, and a wavelength-multiplexed transmission system in which optical signals having different wavelengths are multiplexed to increase the transmission capacity is used.
In the wavelength-multiplexed transmission system, the deviation of the signal strength of each wavelength is very significant. That is, large deviation of a signal strength of each wavelength deteriorates the optical SNR in a receiver. Therefore, it is necessary to reduce the deviation of the signal strength of each wavelength. To reduce the deviation of the signal strength of each wavelength, a compensator having an inverse wavelength characteristic to offset the deviation of the signal strength of each wavelength is often used.
The deviation of the signal strength of each wavelength can be caused by, for example, the wavelength characteristic of the loss of a transmission line fiber by Rayleigh scattering and stimulated Raman scattering, the gain wavelength characteristic of an erbium-doped optical fiber amplifier (hereinafter referred to as an EDFA having an erbium-doped optical fiber as its amplifying medium hereinafter referred to as an EDF), etc. When the wavelength characteristic of the loss of the transmission line fiber is compensated for, a tilt compensator, etc. for compensation only for the tilt element is normally used with the linear wavelength characteristic taken into account. When the gain wavelength characteristic of the EDFA is compensated for, the wavelength characteristic is complicated in most cases. Therefore, a device referred to as a gain equalizer having a complicated wavelength characteristic is commonly used. The gain equalizer can be generated by, for example, the technology disclosed by Japanese Patent Publication No.10-285113 of generating a gain equalizer having an inverse wavelength characteristic to the gain wavelength characteristic by combining an etalon filter, etc.
However, the loss wavelength characteristic of a transmission line fiber depends on the type and length of fiber, the strength of a signal light, etc., and the gain wavelength characteristic of the EDFA depends on the composition of an EDF, the average inversion distribution coefficient in the longitudinal direction of the EDF, the length of the EDF, etc. Therefore, the deviation of the signal strength of each wavelength depends on the design of the transmission line and the EDFA. As a result, the wavelength characteristics of the necessary tilt compensator and gain equalizer are different from each other, and there are an enormous number of fixed compensators.
The present invention has been developed to solve the above mentioned problems, and aims at providing a wavelength characteristic variable apparatus capable of freely varying wavelength characteristics with a simple configuration of the apparatus.
The wavelength characteristic variable apparatus according to the present invention includes an optical filter unit having a desired wavelength characteristic, and a reflection unit capable of turning back an incident optical beam and varying the number of times the optical beam is input into the optical filter unit. By varying the number of times the optical beam is input into the optical filter unit, the wavelength characteristics can be effectively varied.
According to the present invention, a desired wavelength characteristic is assigned to an optical beam by inputting the optical beam into an optical filter unit formed by an optical filter having a desired transmission or reflection wavelength characteristic. Especially, by varying the number of times the optical beam is input into the optical filter unit, the wavelength characteristic assigned to the optical beam is accumulated, and the level of the wavelength characteristic of the optical filter unit can be varied. Furthermore, by providing a plurality of optical filters having different wavelength characteristics for an optical filter unit, the accumulated wavelength characteristics can be provided for the optical beam. Therefore, an optional wavelength characteristic can be logically assigned to the optical beam.